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Re: [Qemu-devel] [RFC 1/4] [UNTESTED] xen: Don't force has_dynamic_sysbu

From: Juergen Gross
Subject: Re: [Qemu-devel] [RFC 1/4] [UNTESTED] xen: Don't force has_dynamic_sysbus on machine class
Date: Fri, 24 Mar 2017 14:59:41 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 24/03/17 14:50, Eduardo Habkost wrote:
> On Fri, Mar 24, 2017 at 01:27:31PM +0100, Juergen Gross wrote:
>> On 24/03/17 12:10, Eduardo Habkost wrote:
>>> On Fri, Mar 24, 2017 at 11:24:31AM +0100, Juergen Gross wrote:
>>>> On 24/03/17 11:09, Peter Maydell wrote:
>>>>> On 24 March 2017 at 08:23, Juergen Gross <address@hidden> wrote:
>>>>>> On 23/03/17 22:28, Eduardo Habkost wrote:
>>>>>>> The xen-backend devices created by the Xen code are not supposed
>>>>>>> to be treated as dynamic sysbus devices. This is an attempt to
>>>>>>> change that and see what happens, but I couldn't test it because
>>>>>>> I don't have a Xen host set up.
>>>>>>> If this patch breaks anything, this means we have a bug in
>>>>>>> foreach_dynamic_sysbus_device(), which is supposed to return only
>>>>>>> devices created using -device.
>>>>>>> The original code that sets has_dynamic_sysbus was added by
>>>>>>> commit 3a6c9172ac5951e6dac2b3f6cbce3cfccdec5894, but I don't see
>>>>>>> any comment explaining why it was necessary.
>>>>>> xen-backend devices are created via qmp commands when attaching new
>>>>>> pv-devices to a domain. They can be dynamically removed, too. Setting
>>>>>> has_dynamic_sysbus was necessary to support this feature.
>>>>> This seems like it ought to be handled by marking the xen-backend
>>>>> devices as being ok-to-dynamically-create somehow, not by marking
>>>>> the machine as supporting dynamic-sysbus (which it doesn't).
>>>>> Maybe we don't have the necessary support code to do that though?
>>>> When writing the patches I couldn't find a way to do it differently.
>>>> OTOH I'm not so deep in qemu internals I'd be able to add the needed
>>>> support.
>>>> I'd be happy to test any patch, though.
>>> If xen-backend devices are created via QMP commands, then
>>> has_dynamic_sysbus is (currently) really needed, although I would
>>> have preferred to set it on all x86 machines instead of changing
>>> MachineClass::has_dynamic_sysbus outside class_init.
>>> But with the new whitelist implemented by this series, we could
>>> simply include xen-backend in the whitelist for the machines that
>>> can be used with Xen, and get rid of xen_set_dynamic_sysbus().
>>> I assume plugging/unplugging xen-backend devices apply to both
>>> xen{pv,fv} and pc,accel=xen, right? Do we need to make it work
>>> with "-machine none,accel=xen" and "-machine isapc" too?
>> AFAIK -xenpv, -xenfv and -pc,accel=xen are the only machine types
>> to support. Wouldn't it make sense to do the whitelisting in
>> xen_be_register_common() in spite of setting has_dynamic_sysbus?
> It would, but that would mean we would make the whitelisting
> mechanism more complex: in addition to the static
> per-machine-class whitelist, we would need a runtime whitelist.
> This would make the interface for querying available/supported
> device types more complex and messier, and I would like to avoid
> that.

Okay, understood. And I suppose you don't want to add the Xen
devices to the per-machine-class whitelist (after all my patch
did the same with the has_dynamic_sysbus flag) on demand.

Either way is fine with me.


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